The present invention relates to methods, usable in a facsimile-scanner, for converting a video signal obtained by scanning an original point by point into a two-level signal by means of a comparison with a dynamic threshold signal, of the kind in which first and second companion signals for the video signal are generated by means of generators controlled by the video signal and the dynamic threshold signal is derived from the companion signals. Hereinafter, such a method will be referred to as "of the kind described".
In black and white facsimile transmission, an original to be copied is scanned point by point and line by line in a scanning device by an opto-electronic scanning member and the brightness information from the original is converted into a video signal.
The original may be a printed or typed document, a hand-written text or a graphic representation, in which cases both the background of the original (paper) and also the information applied to it may be white, black, grey or coloured.
The video signal obtained by scanning the original is amplified, converted in a subsequent evaluating circuit into a digital video signal (a black/white signal) and is passed to a reception device along a transmission channel. The reproducing member of the reception device is controlled by the video signal and produces the required copy of the original.
When a white area of the original is scanned, the scanning member emits a video signal of high amplitude, while when a black area of the original is scanned it emits a video signal of low amplitude and in the case of a grey or coloured detail in the original it emits a video signal of medium amplitude. Signals of medium amplitude also occur when narrow lines are scanned in the original.
In the evaluating circuit, the video signal is continuously compared with a threshold signal and it is decided whether a video signal of given amplitude is to be evaluated as "white" or "black" and whether it is to be converted to the white value or black value of the digital video signal.
The correct evaluation of signals of medium amplitude presents considerable problems.
When the background of an original has white and coloured regions which also contain information, there is not very much contrast between the information and the background. In this case, because of its limited resolving power, the scanning member emits a video signal with only slight variations in amplitude but this signal must still be recognised and correctly evaluated at the time of signal conversion.
A reduction in contrast which hampers the interpretation of the signal also occurs when the background varies in brightness from original to original or when the brightness of the background alters within the original, such as may be the case when scanning yellowed paper.
Since the scanning member detects both alterations in the brightness of the background and also alterations in brightness caused by information in the original, a further problem which occurs at the time of the signal conversion is the correct interpretation of the proportions of the video signal attributable to background brightness and to information brightness.
It is already known from U.S. Pat. No. 3,159,815 to compare the video signal with a constant threshold signal for digitising purposes.
With this so-called "constant threshold" the evaluation of a coloured original which can be performed is very poor.
If for example an original with a coloured background and black or white information is being scanned, and if in principle the decision taken with video signals of medium amplitude is for "black", then black information on a coloured background will be lost. If on the other hand the decision is exclusively for "white", then white information on a coloured background will be ignored.
It is true that an operator can set the threshold signal to give a favourable result before the scanning proper but an optimum setting which involves no loss of information is not possible.
By choosing a suitable threshold signal it is also possible to remedy differences in background brightness from original to original. However, since the threshold signal is then constant during the actual scanning operation, no allowance is made for slow changes in background brightness within the original, as a result of which the DC voltage component of the video signal shifts in relation to the constant threshold and the video signal may be misinterpreted.
To compensate for slow variations in background brightness, it is known from German Auslegeschrift No. 1171464 to compare the video signal with a threshold signal which is proportional to the mean background brightness of the original. This so-called "sliding threshold" is slowly readjusted when there are changes in the brightness of the background, thus automatically compensating for differences in the whiteness of the paper within the original.
The readjustment of the threshold signal to follow the change in the brightness of the background takes place with a long time-constant. The threshold signal therefore adjusts itself to the new value only after several lines have been scanned in the original. Rapid changes in the brightness of the background, such as occur in originals containing information on a coloured background, cannot be allowed for by a sliding threshold signal.
It is therefore an object of the invention to provide a method of converting a video signal into a two-level video signal of the kind described, but which overcomes or minimises the disadvantages mentioned and by means of which coloured originals and originals containing fine details can be converted into a purely black and white copy without any substantial loss of information.